Few of the known escapements combine efficiency and accuracy:                Swiss lever escapements are reliable and robust, but with low output because the impulse passes through a friction plane, via pallets;        coaxial or Daniels escapements have bitangential impulses, and consequently are more efficient, but the components and assembly thereof are complex and expensive;        frictional rest escapements do not have a lever, they include a single wheel giving a single impulse to the balance, but the wheel is in prolonged contact with the balance, thereby wasting energy through friction and disruption to the regulating member;        detent escape mechanisms are reputed to be the most accurate, and have long been reserved for marine chronometers. They have a single impulse, in a single oscillation of the balance and are the mechanisms offering the best mechanical efficiency.        
The efficiency of detent escape mechanisms is greater than that of lever escapements, because the escape wheel only imparts an impulse to the balance once per oscillation, during which the escape wheel pivots through one angular step. Consequently, energy wasted due to the inertia of the escape wheel occurs once per oscillation, as opposed to once per vibration in lever escapements.
The use of detent escapements in wristwatches is more complex, because of the sensitivity of such escapements to shocks.
Most detent escapements include a detent lever also called a brake-lever, which includes locking means, formed by at least one locking stone for holding the escape wheel in the locked position. This lever also includes an actuating finger or beak, which may also be formed by the end of a spring secured to the lever, to cooperate with a discharging pallet comprised in a roller integral with the balance, or even with a notch in said roller.
The balance also carries, generally on another roller, an impulse pallet. When the escape wheel is released, the barrel torque is transmitted by the escape wheel to the impulse pallet, which maintains the alternate pivoting movement of the balance.
In short, the escape wheel is released when the balance rollers pivot in one direction, and it remains locked when the rollers pivot in the other direction. Consequently, the impulse is only given once per oscillation.
In the event of shocks, particularly lateral shocks, if the balance is made to pivot outside its normal amplitude, one tooth of the escape wheel may leave the locking stone, and unlocking and impulses occur twice in the same vibration. The effect of this phenomenon, called “tripping”, is to distort the isochronism of the oscillator.
U.S. Pat. No. 40,508 in the name of James Stephenson, dated 1863, discloses an escapement according to the conventional model, wherein the lever beak cooperating with the discharging pallet is formed by a strip spring fixed to the detent escapement.
U.S. Pat. No. 180,290 in the mane of F. Herman Voigt, dated 1876, also discloses a spring mechanism, this time hooked to the felloe of the balance.
CH Patent No. 3299, in the name of Emile James, dated 1891, discloses a mechanism wherein the detent-lever is carried by a mainspring, and wherein the escape wheel is held on the lock in the event of shaking or a shock. A first roller of the balance includes a lifting piece which cooperates with a strip spring secured to the detent lever. The latter has a beak which, when the lifting piece lifts the detent, cooperates with a notch in a second balance roller, parallel to the first. In the event of shock, the spiral spring on which the detent is mounted tends to keep the escape wheel pressed against the locking stone. This mechanism can be applied to detent-spring escapements.
Detent escapements did not change very much during the twentieth century.
The Daniels escapement, known from EP Patent No. 0 018 796 in the name of George Daniels and dated 1984 will, however, be cited. This escapement is slightly different from conventional detent escapements, but it includes the main characteristics, notably a lever cooperating, on the one hand, with a balance pin, and on the other hand with a first external escape wheel, which imparts the impulse to the balance, through two entry and exit pallets, and with a second escape wheel coaxial to the first via a separate impulse pallet.
It was not until 2005 that new detent escapements dedicated to wristwatches were disclosed.
EP Patent No. 1 538 490 in the name of MONTRES BREGUET SA discloses an escapement of this type, including a lever-detent with a locking stone and a first actuating finger. The latter cooperates with a second actuating finger mounted on a very long resilient member pivoting on the balance roller that carries the impulse pallet. This second finger can drive the first finger when the roller is pivoting in a first direction to actuate the lever, and can bypass the finger without driving it when the roller is pivoting in the opposite direction. This resilient member includes an aperture that moves around a first pin carried by the roller, which also carries a second pin that may or may not cooperate with the resilient member, depending upon the level of tension or extent to which said member is let down. In another embodiment, the resilient member of this escapement is on the lever, and not on the roller. This resilient member may consist of a spiral spring, or a serpentine spring.
Various improvements and variants follow, in Patents EP 1 538491, EP 1 544689, EP 1 708046 and EP 1 708047, in the name of MONTRES BREGUET SA.
The first, EP 1 538491, discloses a mechanism which has no resilient member on the detent, but has a resilient member carrying the second actuating finger on the roller. Said roller is provided with a cam notch in which a beak, located at the end of a feeler arm of the detent lever, moves.
EP Patent No. 1 544689 discloses a similar mechanism, but with no resilient member, wherein the second actuating finger is positioned opposite the cam notch and cooperates with one or other side of the first finger of the detent-lever depending upon the direction in which the roller is pivoting, to drive or hold the locking stone. The function of the second actuating finger is to re-engage the locking stone in the escape wheel, and no feeler arm with a beak is used.
EP Patent No. 1 708047 discloses a lever including an arm carrying both a first finger, for cooperating with the second actuating finger, and a feeler with a beak cooperating with a cam notch similar to the preceding ones. When the balance and rollers pivot in a first direction, the first finger drives the second finger to unlock the locking stone(s) from the escape wheel. The feeler arm beak is then driven by a rising flank of the cam notch to re-engage the locking means in the escape wheel. When the balance pivots in the opposite direction, the first finger drives the second finger to keep the locking means engaged in the escape wheel. When the first and second fingers meet, in whatever direction the rollers are pivoting, a natural rotating force is generated on the lever arbour. This encounter does not generate any risk of breaking the mechanism. No resilient member or stop pins are necessary. In a particular embodiment, this mechanism includes two juxtaposed locking stones including contiguous but non-aligned locking faces, which enables the tip of the escape wheel tooth to be housed on a locking line at the junction of the locking faces, creating a draw effect which dispenses with the need for any stop pin. The locking face of the locking stone closest to the escape wheel rises up before the tooth and prevents it from continuing on its way. In this total lock position of the escape wheel tooth, the beak of the feeler arm moves away from the periphery of the roller leaving the balance completely free to complete the first vibration. This design makes the escapement resistant to shocks. Indeed, a shock returns the beak onto the periphery of the corresponding roller, without however unlocking the locking stones, since the return of the tooth onto the locking line occurs immediately owing to the draw effect. When, afterwards, during the return movement of the balance in the opposite direction towards the end of the second vibration, the first finger and the second finger come into cooperation, they create torque in the detent lever around its pivot axis, causing a slight recoil of the escape wheel tooth, before the tooth returns to the locking line in an anti-draw effect, when the fingers separate.
EP Patent No. 1 708046 discloses an improvement applicable to these various versions, in the form of a safety finger fixed to the roller, and arranged to cooperate with the teeth of the escape wheel and lock said wheel if the impulse pallet is accidentally released from the toothing of the wheel. This arrangement prevents the escape wheel from racing in the event of a shock which results in the direction of rotation of the rollers being reversed at the precise moment of the wheel impulse. The collision of one tooth of the wheel with this safety finger locks the wheel and returns the rollers to the proper direction of rotation.
These Patents therefore proposed both simplifying and making the detent escapement more secure.
Other documents have proposed other solutions.
Thus, EP Patent No. 1 522001, in the name of Detra SA and Patek Philippe SA also published in 2005 proposes an escape mechanism with locking parts and toothed wheels with gaps in the toothing. The first wheel set is subjected to a periodic torque, obtained for example by a rotor mounted in a stator. This first wheel set includes, on the one hand, in a basic plane, a first wheel with gaps in the toothing over the periphery thereof, and on the other hand, in a second plane, a first brake-lever including several teeth and able to lock a release lever comprised in a balance roller, when the balance pivots in a first direction. Depending upon its position, this first wheel set cooperates with a second wheel set, either via the first brake-lever or via the first wheel thereof. This second wheel set includes, in the basic plane, a second wheel with toothing gaps, in the second plane, a shaped part which includes several fingers and can lock the balance roller release lever in a second direction of pivoting opposite to the first. The second wheel set further includes a locking part in a first plane parallel to the preceding planes. Depending upon its position, this second wheel set cooperates, either via the locking part, or via the second wheel thereof, with an escape wheel, which includes, in the basic plane, a toothed wheel with toothing gaps, and in the first plane, an impulse wheel, which receives a continuous mechanical torque such as that from a barrel, similar to a conventional escape wheel, and can cooperate with the impulse lever comprised in the balance roller, to maintain the oscillating movement of the balance. Depending upon the respective angular positions of the various wheel sets, the locking parts, or shaped parts, or teeth, cooperate with each other, such that the device has four stable locking positions for each revolution of the first pin, between which it has the same number of unlocking positions. The combination of two locking means and two unlocking means for the mechanical torque, and the particular sequence imposing an unlocking operation between two locking operations prevent any racing or tripping in the event of a shock to the mechanism. This mechanism is complex, relatively expensive and extends over several planes, which gives it significant thickness.
EP Patent No. 1 770 452 in the name of Peter Baumberger is an improvement of the former Voigt U.S. Pat. No. 180,290 devised to minimise the requirement for space, and it discloses a conventional detent escapement with a detent lever that pivots and is returned by a spiral spring, one arm of the lever carries one end of a strip spring, the other end of which is held abutting on a stop member carried by another arm of the lever, and is arranged to cooperate with a unlocking stone integral with a small roller of the balance. Another arm of the lever, beyond a locking stone, includes a finger which cooperates with the periphery of this small roller, and in particular with a truncated portion forming a cam, at a lower level than that of the strip spring. A large balance roller conventionally carries an impulse pallet, preceded by a first recess, and followed by a second recess, to allow the locking stone to be unlocked when the locking stone pivots the detent lever. The selection of a particular geometry, both as regards the position of the locking stone and the impulse pallet in quasi-symmetry with the line at the centres of the escape wheel and the balance during the locking phase, and the fork formed by the finger and the free end of the strip spring, limit the disruptive effect linked to the detent inertia on the balance oscillations. The amplitude of the pivoting movements of the detent, in the event of shock, is limited by the interaction of the locking stone and the large roller. In a complementary embodiment, this mechanism includes an anti-trip lever, in proximity to the small roller, pivotably mounted on the movement between two stable end positions maintained by a jumper spring on stop members with which a first end can cooperate and the second fork-shaped end of which interacts with the discharging pallet: each time the discharging pallet passes into the fork it exerts pressure to tip the anti-trip lever from one stable position to the other. The fork thus forms two stops for the small roller in the event of any tripping, and prevents the balance from pivoting through more than one revolution.
EP Patent No. 1 860 511 in the name of Christophe Claret SA discloses a movement with a moveable bridge, providing protection for a detent escapement against lateral shocks. This moveable bridge carries the sprung-balance pivot axis, the escape wheel pivot axis, the detent pivot axis and part of the gear train. It is pivoted elastically on the arbour of one of the gear train wheels, for example the seconds wheel. Forces, such as a lateral shock, capable of unlocking the locking stone, then drive the entire moveable bridge and the relative positions of the detent and the escape wheel are maintained. This ensures constant operation of the escapement. The moveable bridge may also be dampened by a dampening system which dissipates part of the energy due to the shock.
EP Patent No. 2 221677 in the name of Rolex SA discloses an innovative detent escapement, which includes an inertia mass that can be pivoted relative to the balance roller under the effect of acceleration of the balance. This inertia mass carries the unlocking finger, whose function is to cooperate with the detent lever finger. The mass is pivoted on an off-centre arbour of the roller and the angular clearance thereof is limited by the travel of a pin in a elongated hole, which corresponds to two stable positions, i.e. one in each direction of pivoting of the balance. Thus, depending upon the acceleration of the balance, the unlocking finger either does or does not protrude relative to the balance roller, and therefore is, or is not able to engage the detent lever finger. The unlocking finger does not, therefore, have to overcome the resistance of any elastic member to pass the obstacle of the detent lever finger during the vibration in which the balance does not receive an impulse for its oscillating movement, since the unlocking finger is withdrawn and remains set back from the edge of the roller. There is no loss of energy or disruption to the oscillation period of the balance.
CH Patent Application No 700 091 in the name of Christophe Claret SA discloses a detent escapement with a detent lever, which is pivotably mounted on a spiral spring and cooperates at the other end with a first strip spring embedded in proximity to the pivot. The balance roller includes two distinct discharging pallets. A wheel set, arranged on the opposite side of the escape wheel relative to the detent lever, carries a pivoting cam, which holds a cam strip spring and is returned towards the detent by a spiral spring onto a stop position. This cam is arranged for making the cam strip spring cooperate, either in a first state, with the end of the lever carrying the strip spring, or in a second state, with the discharging pallets of the balance. The first discharging pallet is arranged to cooperate with the first strip spring and actuate the detent when the first pallet encounters the first strip in a first direction, and to cooperate only with the first strip without actuating the detent when it encounters the first strip in the opposite direction. When the cam is in the first state it cooperates with the detent to limit the movements thereof. The second discharging pallet is arranged for changing the cam to the second state in which the detent is free to perform its unlocking operation and release the escape wheel tooth from the locking stone. The two discharging pallets are close and arranged such that the cam is brought into its second state just before the detent performs the unlocking operation. The spiral cam return spring, which is stronger than the cam strip spring, tends to return the cam to its first state. Thus, in the first state thereof, the cam is positioned so as to oppose any inadvertent movement of the detent which could result in inadvertent unlocking of the locking stone, and the escapement is less sensitive to the effects of a shock. Adjustment of this mechanism is complex, since it depends upon the features peculiar to the springs comprised therein, of which there are at least three.
EP Patent No. 2 224 292 in the name of Rolex SA discloses a direct impulse escapement, particularly a detent escapement. The detent lever is arranged in a particular manner, pivoting between two stop members. Facing the escape wheel, it has a finger including, in succession, a stop surface used as the locking stone, a safety surface which, depending upon the pivotal position of the lever, either interferes or does not interfere with the escape wheel trajectory, and a sliding surface which forces the lever to tip, when the escape wheel is pivoting, so as to return the sliding surface and thus the stop surface to the area of interference with the escape wheel, to stop said wheel pivoting. The balance roller conventionally includes an impulse pallet and an unlocking finger. During the vibration in a first pivoting direction of the balance, the lever is in a first stopped pivotal position which allows the unlocking finger to pass, whereas in the other vibration in the other pivoting direction, the lever is pivoted into another stop position and encounters the unlocking finger at an elastic unlocking element comprised in said lever. The elastic travel of this elastic unlocking element allows the balance to continue its travel and the impulse pallet passes between two adjacent teeth of the escape wheel. Shortly afterwards, the balance is stopped by the balance spring thereof and pivots in the opposite direction. During this elastic travel, the lever remains butting against the stop member and the stop surface of the lever slides over the escape wheel tooth which is kept stopped. The safety of this mechanism is ensured by the arrangement of the lever finger, with one stop surface and one sliding surface which alternately run into the trajectory of the escape wheel toothing. The length of the safety surface between the stop surface and the sliding surface corresponds to the angle travelled by the wheel to communicate the drive energy to the balance, to prevent the premature return of the stop element into the trajectory of the wheel, which provides additional security. Part of the energy from the barrel is, however, consumed in friction during the sliding phase.
It is clear that these various mechanisms are complex, require many components and may be difficult to adjust.